With reference to FIG. 1, a conventional VIPA 1 includes two parallel surfaces, a first highly-reflective surface 2, which has a highly reflective coating 3 thereon, and a second partially-reflective surface 4 with a partially reflective coating 5 thereon. The first highly-reflective surface 2 also has an input, anti-reflection zone 6 with an anti-reflection coating 7, which abuts the highly-reflective coating 3, with a transition zone 8 therebetween.
Light 9 entering the VIPA 1, see FIG. 2, through the input zone 6, reflects back and forth across a gap formed by substrate 10 between the highly and partially reflective surfaces 2 and 4, respectively, gradually leaking out through the partially reflective surface 4. Because the two reflective surfaces 2 and 4 are highly parallel, the output beams have a well-defined phase relationship, which enables the use of the VIPA 1 as a spectrometer, dispersion compensator, multiplexer/demultiplexer or filter.
One of the keys to the operation of the VIPA 1 is a narrow transition zone 8 between the highly reflective surface 2 and the input zone 6. Conventionally, the width of the transition zone 8 is controlled with some sort of mask during the deposition of the highly-reflective coating 3 and the anti-reflection (AR) coating 7 by a number of coating processes, e.g. electron beam evaporation, sputtering, etc.
The mask could be a metal foil held in contact with the highly-reflective surface 2 during the deposition of the highly-reflective coating 3, or a photoresist that is exposed and developed during assembly. After coating, with the highly-reflective coating 3, the mask is removed, which may be a chemical removal process in the case of a photoresist. In either case, (or with any other masking technique), the width of the transition zone 8 is affected by the geometry of the mask, including the straightness of the mask, the thickness of the mask, the contact of the mask with the highly-reflective surface 2, and the deposition process shadowing of the edge of the highly-reflective surface 2. Typically, mechanical masking will result in a transition zone 8 with a width w of 50 μm or more.
For the input light 9 to be efficiently coupled into the VIPA 1, all of the light 9 must avoid the transition zone 8 during entry and after its first bounce from the partially reflective surface 4. The width w of the transition zone 8 thus sets a minimum entrance angle from a normal to the input zone 6 into the VIPA 1. Because VIPAs typically rely on a hundred or more bounces, a large transition zone 8 requires a large entrance angle resulting in a wider distance between bounces, and consequently increases the length of the VIPA 1 and the size of the associated optics in order to achieve the maximum spectral resolution of the device. Furthermore, the larger length increases the difficulty in manufacturing the VIPA 1, resulting in a higher cost.
An object of the present invention is to overcome the shortcomings of the prior art by providing a VIPA with a smaller transition zone to enable a smaller entrance angle for incoming light.